1. Field of the Invention
This invention relates to a radiation imaging method, a radiation imaging apparatus, a computer program and a computer-readable recording medium, and particularly is suitable for use for correcting the defect of a function in the pixel of a radiation detector.
2. Related Background Art
As a radiation imaging apparatus for imaging a radiation image transmitted through an object, use has heretofore been made of an imaging apparatus called a screen-film system (S/F system) in which an intensifying screen for converting radiation into fluorescence and photographic film are brought into close contact with each other.
An X-ray image intensifier-television (XII-TV) imaging apparatus for imaging a radiographic image with a combination of a fluorescent material, an image intensifier, an optical system and an image pickup tube has also been used.
The former has been primarily used in general radiography, and the latter has been primarily used in fluoroscopy or angiography.
Recently, a digital imaging apparatus having the function of outputting a digital image has been available from the demand for the digitization of an image.
A computed radiography apparatus using an imaging plate for accumulating a radiation image as a latent image, laser-scanning this imaging plate to thereby excite the latent image, and reading fluorescence produced thereby by a photomultiplier has also been used in general radiography substituting the aforedescribed screen-film systems.
Also, II-DR (image intensifier-digital radiography) apparatus using a solid state image pickup element such as a charge coupled device (CCD) instead of an image pickup tube has also been available in fluoroscopy and angiography.
Both of such a computed radiography apparatus and the I.I.-DR imaging apparatus have the function of outputting a digital image, and begin to contribute to the digitization of a medical image.
Further, recently, there has been put into practical use a digital imaging apparatus for directly digitizing a radiation image without the intermediary of an optical system or the like, by the use of a so-called flat panel detector (FPD) in which a fluorescent material and a large-area amorphous silicon sensor are brought into close contact with each other.
There has also been likewise put into practical use an FPD for converting radiation into an electron by the use of amorphous selenium, lead iodide (PbI2) and mercury iodide (HgI2), and detecting the electron by a large-area amorphous silicon sensor.
Both of these FPDs are very similar in principle to each other, and the former is called an indirect type and the latter is called a direct type.
In the FPD, a portion for capturing an X-ray and converting it into a secondary quantum is called a primary sensor, and in the indirect type, it is a fluorescent material, whereas in the direct type, it is amorphous selenium or the like.
On the other hand, a portion for detecting a secondary quantum is called a secondary sensor, and in the indirect type, it is an optical sensor provided with an amorphous silicon thin film transistor (TFT) array, a light-receiving portion and a charge accumulating portion, whereas in the direct type, it is a charge sensor provided with an amorphous silicon TFT array and a charge accumulating portion.
These FPDs are theoretically capable of imaging not only a still image but also a moving image and therefore are expected as the digital imaging apparatuses of the next generation.
The principle of the reading operation of the secondary sensor used in the FPD will now be described briefly with reference to FIG. 7 of the accompanying drawings. In FIG. 7, for simplicity, a secondary sensor comprising nine pixels is shown.
In FIG. 7, the reference characters 71a-71i designate photoelectric converting portions for converting fluorescence into an electron, the reference characters 72a-72i denote thin film transistors (TFTs) for forwarding the electrons produced in the photoelectric converting portions, the reference numeral 73 designates a bias line for giving a bias voltage to the photoelectric converting portions 71a-71i, the reference characters 74a, 74b and 74c denote gate lines for transmitting a switching signal to the TFTs 72a-72i, the reference characters 75a, 75b and 75c designate signal lines for forwarding electrons passed through the TFTs 72a-72i, the reference numeral 76 denotes a read-out device for selecting a signal line from among the signal lines 75a, 75b and 75c and amplifying a signal electron, the reference numeral 77 designates an analog-to-digital converter (A/D converter) for converting the amplified analog signal into a digital signal, and the reference numeral 78 denotes a gate driving unit for controlling the switching operation of the TFTs 72a-72i. 
In FIG. 7, when radiation is exposed to a fluorescent material, not shown, covering the whole surface of the pixel of the secondary sensor, the fluorescent material emits fluorescence proportional to the intensity thereof. The photoelectric converting portions 71a-71i capture this fluorescence and convert it into a signal electron. When the gate driving unit 78 makes the gate line 74a High, a row of TFTs 71a-71c connected to this gate line 74a all become ON. Thereupon, the signal electrons accumulated in the photoelectric converting portions 71a-71c are forwarded to the signal lines 75a, 75b and 75c, respectively.
The read-out device 76, when it selects the signal line 75a, amplifies and reads the signal electron forwarded to this signal line 75a. The signal electron read by the read-out device 76 is converted into a digital signal by the A/D converter 77.
Subsequently, the read-out device 76 selects the signal lines 75b and 75c in succession, and reads the signal electrons forwarded to the respective signal lines 75b and 75c in succession.
By this operation, the signal electrons of three pixels corresponding to a row connected to the gate line 74a are read and converted into a digital signal.
Next, the gate lines 74b and 74c are successively selected, and the signal electrons of three pixels corresponding to a row connected to the gate lines 74b and 74c, respectively, like the pixels connected to the gate line 74a, are successively read and converted into digital signals.
FIG. 8 of the accompanying drawings shows an equivalent circuit simulating a pixel of a metal insulator semiconductor (MIS) type secondary sensor.
In FIG. 8, the reference numeral 71 designates a photoelectric converting portion, the reference numeral 72 denotes a TFT, the reference numeral 73 designates a bias line, the reference numeral 76 denotes a read-out device, the reference numeral 82 designates an upper electrode (D electrode) for transmitting a bias voltage to the photoelectric converting portion 71, the reference numeral 83 denotes an n+dope layer being of the same potential as the upper electrode 82 and blocking the injection of a hole into a-Si intrinsic semiconductor i layer 84, the reference numeral 84 designates the a-Si intrinsic semiconductor i layer for effecting photoelectric conversion, the reference numeral 85 denotes an insulating layer for blocking the movement of an electron and a hole, and the reference numeral 86 designates a lower electrode (G electrode).
The secondary sensor used in the FPD comprises millions of pixels, but the characteristic of each pixel differs delicately. Particularly, an important characteristic as an image sensor is a dark current (distribution) and a sensitivity characteristic (distribution). A sensitivity characteristic (distribution) also exists in the primary sensor.
So, in the FPD, the step of correcting these characteristics (correcting step) is carried out so as to provide a sensor in which the characteristics of the respective pixels are substantially uniform. This correcting step is substantially common to both of the aforedescribed indirect type and direct type. Description will hereinafter be made of methods of correcting the dark current and the sensitivity characteristic.
The method of correcting the dark current will first be described. Here, the dark current is a current measured when there no input to the sensor, and it is to be understood that it comprises a random component and a stationary offset component.
Assuming that the dark current does not depend on a sensor input, an image when a signal is not inputted to the sensor is subtracted from an image when a signal has been inputted to the sensor, whereby the correction of the offset component of the dark current differing in each pixel becomes possible.
A radiation image when a signal has been inputted to the sensor is defined as X (x,y) and a dark current image measured immediately thereafter is defined as Dx(x,y), a primary dark current corrected image Cx(x,y) after the dark current has been corrected is represented by the following expression (1). In expression (1), x,y is the address of two-dimensionally arranged pixels.Cx(x,y)=X(x,y)−Dx(x,y)  (1)
The correction of the sensitivity characteristic will be described hereafter. The sensitivity correction is sometimes called calibration. The sensitivity correction is the step of correcting the unevenness of the sensitivity of the pixels constituting the sensor, and generally the sensitivity of the primary sensor and the sensitivity of the secondary sensor are corrected at a time.
Assuming that the sensitivity is stationary, the image when a signal has been inputted to the sensor is divided by an image when a uniform input has been given to the sensor, whereby the correction of the sensitivity differing in each pixel becomes possible.
An image when a uniform input has been given to the sensor is defined as Cw(x,y), and a radiation image component and a dark current image component included in the image Cw(x,y) when the uniform input has been given to the sensor are defined as W(x,y) and Dw(x,y), respectively, a sensitivity-corrected image C(x,y) is represented by the following expression (2). In expression (2), the upper line represents an average value.
                              C          ⁡                      (                          x              ,              y                        )                          =                                                            Cw                ⁡                                  (                                      x                    ,                    y                                    )                                            _                                      Cw              ⁡                              (                                  x                  ,                  y                                )                                              ·                      Cx            ⁡                          (                              x                ,                y                            )                                                          (        2        )            
When expression (1) is used, expression (2) is represented as the following expression (3).
                              C          ⁡                      (                          x              ,              y                        )                          =                                                                              W                  ⁡                                      (                                          x                      ,                      y                                        )                                                  -                                  Dw                  ⁡                                      (                                          x                      ,                      y                                        )                                                              _                                                      W                ⁡                                  (                                      x                    ,                    y                                    )                                            -                              Dw                ⁡                                  (                                      x                    ,                    y                                    )                                                              ·                      {                                          X                ⁡                                  (                                      x                    ,                    y                                    )                                            -                              Dx                ⁡                                  (                                      x                    ,                    y                                    )                                                      }                                              (        3        )            
In a radiation imaging apparatus, it is usual to apply uniform radiation for sensitivity correction. However, at a dosage level utilized in radiation image diagnosis, a quantum noise proportional to the square root of the number of radiation quanta is superimposed on the radiation image. This quantum noise is unavoidable in principle and therefore, the quantum noise is of course also superimposed on the image Cw(x,y) when a uniform input has been given to the sensor. That is, it is feared that the accuracy of sensitivity correction is aggravated by the quantum noise superimposed on the image Cw(x,y) when a uniform input has been given to the sensor.
So, generally, for the purpose of sensitivity correction, it is practised to effect the averaging of images obtained by a plurality of exposures, and increase the number of actually effective radiation quanta included in the image Cw(x,y) when a uniform input has been given to the sensor to thereby improve the accuracy of sensitivity correction.
In addition, not all of the millions of pixels constituting the secondary sensor used in the FPD always operate, but a finite number of defective pixels are included therein. An appropriate number of defective pixels are allowed with the influence upon diagnosis, the yield and cost taken into account. The defective pixels of the secondary sensor are caused by the leak due to the malfunctions of the TFTs 72a-72i, or the opening or mutual short-circuiting of the bias line 73, the gate line 74 and the signal line 75. Accordingly, there are diversified defective modes. The outputs of these defective pixels are often not correlated with the outputs of normal pixels. Also, in some cases, the outputs fluctuate at random.
Likewise, a finite number of defective pixels also exist in the primary sensor such as a fluorescent material or amorphous selenium use in the FPD.
In the case of the fluorescent material, there is a defective mode in which a foreign substance gets mixed in a fluorescent material layer and light is attenuated. Also, in the case of columnar cesium iodide (CsI) fluorescent material, there is a defect mode in which sensitivity is locally varied by the abnormality of crystal growth. Further, in the case of amorphous selenium, it is said that there is a defect mode in which a pinhole is produced during evaporation and thousands of volts of bias is short-circuited by the pinhole.
The defective pixels are interpolated by the utilization of the outputs of neighboring pixels surrounding the defective pixels. In the case of such isolated point defect as shown, for example, in FIG. 9 of the accompanying drawings that a defect exists in only the central pixel 90, it is usual that the defect is corrected by the use of the average value of neighboring eight pixels.
It is also possible to effect defect correction adaptively in conformity with the outputs of the neighboring pixels. For example, when it is estimated from the information of the neighboring pixels that the edges thereof overlap the defective pixels, it is also possible to regard the result of a calculation effected so as to sharply reproduce the edges as the outputs of the defective pixels.
The definition of the defect is considered to differ depending on the radiation imaging apparatus, but generally an image when uniform radiation is exposed to the FPD is used. This image is fluctuated by the sensitivity characteristic of the FPD or quantum noise and has finite standard deviation. However, the defective pixels often present an output exceeding the degree of these fluctuations. So, in the image when uniform radiation is exposed to the FPD, there is conceived a method of setting the region of interest (ROI) of e.g. 128×128 pixels, obtaining an average value m in the ROI and standard deviation σ, and defining a pixel of which the output exceeds (m±5σ) as a defect. There is also conceived a method of defining a pixel of which the output exceeds (m±0.2 m) as a defect. There is also conceived a method of likewise detecting a defective pixel by the utilization of a dark current image obtained when radiation is not exposed to the FPD.
It is empirically confirmed that these defects are hardly deteriorated in both number and degree within the life range of the apparatus in an ordinary state of use. So, during the shipment from a factory, a defect map is prepared and by the interpolating process, the output of a defective pixel is substituted for an input estimated to be correct. Also, with a rare increase in defects taken into account, the defect map is renewed during periodical inspection or the like in the market.
In Japanese Patent No. 2712495, there is disclosed a method of pre-storing a defective position attributable to an image intensifier (I.I.) and a television (TV) camera for picking up the output optical image of the image intensifier, and compensating for a diagnostic image on the basis of the stored defective position.
Also, in Japanese Patent Application Laid-Open No. 2001-8106 and Japanese Patent Application Laid-Open No. 2001-8107, there is disclosed a method of registering a defect during the shipment from a factory as an initial defect map, further newly preparing a defect map during periodical inspection or the like, preparing a combined defect map from the two, and effecting the correction of the defective pixel of a diagnostic image (defect correcting step).
These defect correcting steps will now be described with reference to FIG. 10 of the accompanying drawings.
In FIG. 10, the reference numeral 31 designates a defect correction program, the reference numeral 32 denotes an initial defect map produced during the shipment from a factory, the reference numeral 33 designates a QC defect map produced during periodical inspection in the market, the reference numeral 34 denotes a photographed image, the reference numeral 35 designates a logical sum step portion, the reference numeral 36 denotes a combined defect map, the reference numeral 37 designates a defect correcting step portion, the reference numeral 38 denotes an interpolated image, the reference numeral 39 designates a substituting step portion, and the reference numeral 40 denotes an image after processed.
In the initial defect map 32 and the QC defect map 33, normal pixels are indicated as white pixels, and defective pixels are indicated as black pixels. It is because an inspecting method and algorithm for producing the initial defect map 32 and the QC defect map 33 differ that the two do not partly coincide with each other.
Also, in the photographed image 34, defective pixels are expressed as white pixels or black pixels with the fact that the output level of the defective pixels is unspecified taken into account.
Inputs to the defect correction program 31 are the initial defect map 32, the QC defect map 33 and the photographed image 34. In the logical sum step portion 35, the combined defect map 36 is produced by the use of the initial defect map 32 and the QC defect map 33. Subsequently, the photographed image 34 and the combined defect map 36 are inputted to the defect correcting step portion 37, and the pixel values of defective pixels are estimated by the interpolating process or the like, and the interpolated image 38 is produced. Lastly, in the substituting step portion 39, only the defective pixels included in the photographed image 34 are substituted for by the interpolated image 38, and the image 40 after processed is produced, and this produced image 40 after processed is outputted from the defect correction program 31.
Now, we have progressed the analysis of the aforedescribed defective mode and have found that in the conventional definition of defect, defects are excessively counted. For example, we have found that in some defect modes in a fluorescent material, the amount of light is simply attenuated and by correcting sensitivity, it is possible to handle the defective pixels as normal pixels.
Likewise, we have found that in a part of the defect mode of the secondary sensor, the output simply lowers (rises) and by correcting sensitivity, it is possible to handle the defective pixels as normal pixels.
That is, we have found that for example, even among pixels of which the output exceeds (m±5σ) or (m±0.2 m) and which are defined as defects by the aforedescribed definition of defect, there exist pseudo-defective pixels which are not defects in practice.
To discern between the pseudo-defective pixels looking defective but normally usable and the true defective pixels, a precise test is necessary. In this case, for example, a method of changing the radiation intensity level and imaging many times to thereby inspect the linearity of the pixel is effective. A method of repeating the same imaging several times to thereby inspect the stability of the output is also effective.
However, it is difficult to perform such inspection at a place whereat the radiation imaging apparatus is installed. Further, it is difficult to discern the pseudo-defective pixels and the (true) defective pixels by periodical inspection performed by a user. So, it is desirable that the step of classifying the pseudo-defective pixels and the defective pixels be executed before the shipment from the factory.
However, if simple defect inspection is performed in the periodical inspection at the place whereat the radiation imaging apparatus is installed, there is the possibility that these pseudo-defective pixels are extracted as defective pixels and the extracted pseudo-defective pixels are additionally registered in the defect map. In principle, some estimation enters the defect correcting process of substituting the pixel value and therefore, the defect correcting process should be limited to the necessary minimum, and it is not desirable that the pseudo-defective pixels be included in the object of defect correction.
Also, some radiation imaging apparatuses have a specification which does not approve any connected defect. In some defect modes in a fluorescent material, the attenuation of the amount of light extends over several pixels. If this is defined as a defect, not only useless defect correction will increase, but also in some cases, there is the possibility that the sensor itself is judged to be a defective article by mistake.